Signal processor

ABSTRACT

A geophone senses seismic vibrations in the earth and converts them to electrical seismic signals which are amplified and filtered in an active bandpass filter-amplifier having automatic gain control and a 10-40 Hz. passband. The amplified and filtered signal is rectified in a first full-wave rectifier, is subsequently filtered in a circuit having a 1-4 Hz passband. characteristic, and is again rectified in a second full-wave rectifier. The latter rectified signal is summed by an integrator over a 10-second time interval and is applied to a threshold circuit. If the sum signal exceeds a prescribed threshold level, the threshold circuit produces an output signal indicating that the seismic vibrations are burst-type seismic vibrations such as are produced by a man walking in the area of detection of the geophone.

uluteu mares ratent 1 1 3,691,549

amplified and filtered in an active bandpass filter-amplifier havingautomatic gain control and a 10-40 Hz.

[52] US. Cl. ..340/261', 340/258 D passband. The amplified and filteredsignal is rectified 51 Int. Cl. ..G08b 21/00 n a fir fulhwavc rectifier.is subsequently filtered in [58] Field of Search ..340/26l, 258 D, 258R, 15, admit having a Hz p characteristic, and 340 55 6C, 1 27 is againrectified in a second full-wave rectifier. The

latter rectified signal is summed by an integrator over 5 Wilson a I [61Sept. 12, 1972 I 1 SIGNAL PROCESSOR Attorney-Norman J. OMalley, RussellA. Cannon 72 inventor: Dennls L. Wilson, Palo Alto, Calif. and

[73] Assignee: Sylvanh Electric Products, Inc. [57] ABSTRACT [22] Filed:Dec. 2, 1970 A geophone senses seismic vibrations in the earth and .;.j[211 App 94 converts them to electrical seismic signals which are no(56] References Cited a 10-second time interval and is applied to athreshold circuit. If the sum signal exceeds a prescribed UNITED STATESPATENTS threshold level, the threshold circuit produces an outv putsignal indicating that the seismic vibrations are 2: 6/ Llenau et 1 xburst-type seismic vibrations such as are produced by 3,4 FOSICII Ct ..3X a man in the area f detection of the Primary Examiner-John w. Caldwellgmphme' a Assistant Examiner-Michael Slobasky 10 Claims, 8 DrawingFigures r'Z f5 ENVELOPE awn/naps 335 g g 2Z2; 05750012 BAND PASSDETECTOR Gian/ME AMP gfllu- WAV FILTER egFl/LL WAVE J" RECTIFIERPECf/F/EIZ 14 1'2 11 f I l ALA RM THRESHOLD G c 0510c: 0rcr01z 8PATENTEDSEP 12 I972 SEISMIC BAND/ 45$ SENSOR AGC F/U'El? e3, GEOPHO/VE AMR AGC A ENVELOPE' A/VELOPE DETECTOR BAA/D PA$$+ DETECTOR cg, FULL WAVEF/L 75,? eg, FULL WAVE RECTIFIER RECTIF/ER ALA RM THRESHOLD DE/V/V/S L.WILSON" IN VENTOR.

AGE/VT 1 SIGNAL PROCESSOR BACKGROUND OF INVENTION This invention relatesto seismic intrusion detection systems and more particularly to a signalprocessor for such systems.

within the protected area. The geophone converts vibrations in theground to a varying seismic signal voltage that is comprised of spectralcomponents having It is desirable that the system produce an alarmresponse to seismic signals caused by a man walking in the protectedarea without producing false alarms in response to seismic signalscaused by sources such as wind, rain and aircraft. A prior art signalprocessor for accomplishing this function detects the energy in theseismic signal and produces an alarm if the magnitude thereof exceeds aprescribed threshold level. This processor has poor discriminationagainst seismic signals caused by sources of false alarm, however, sincean alarm is produced whenever seismic signals of sufficient energy arecaused by any source. A more sophisticated signal processor generates asignal proportional to the difference between the energies in the lowand high frequency spectral components of a seismic signal fordiscriminating between seismic signals caused by a man walking and thosecaused by sources of false alarm. An alarm is produced only when theamplitude of this difference signal exceeds a prescribed thresholdlevel. Although the false alarm rate of the latter processor isrelatively low, the processor is complex and has a false alarm rate thatis higher than that desirable for certain applications.

An object of this invention is the provision of an improved seismicsignal processor that is relatively simple and provides gooddiscrimination between seismic signals caused by a man walking and thosecaused by wind, rain and aircraft.

SUMMARY OF INVENTION In accordance with this invention, burst seismicsignals caused by a walking man are distinguished from BRIEF DESCRIPTIONOF DRAWINGS FIG. 1 is a block diagram of a seismic intrusion dey tectionsystem embodying this invention;

FIGS. 2A-2F are waveforms useful in explaining the operation of thesystem of FIG. 1; and

FIG. 3 is a schematic circuit diagram of a bandpass filter in the systemof FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1, a processorembodying this invention comprises filter-amplifier 3, a first envelopedetector 5, bandpass filter 7, a secondenvelope detector 9, integrator11 and threshold detector 12 connectedin series between seismic sensor 1and alarm device 14. An automatic gain control (AGC) circuit 4 isconnected across amplifier 3. The waveforms designated by primedreference characters in FIGS. 2 represent the output signals: ofassociated components designated by the same unprimed referencecharacters in FIG. 1. The waveforms in FIGS. 2 illustrate the operationof the system in FIG". 1 in response to seismic vibrations produced inthe ground by a walking man.

The seismic sensor 1 may, by way of example, be a geophone which is anacoustic transducer that is placed in contact with the earth. A seismicvibration in the amplitudes and frequencies related to thesevibrations.,-

earth is detected by the geophone which produces an electrical seismicsignal I (see FIG. 2A) on line 2 comprising all of the frequencycomponents reproduced by the geophone. A walking man periodicallyproduces burst-type seismic vibrations of short duration that result inburst seismic signals 1' that are characterized by strong low frequencyspectral components having frequencies of less than 50 l-lz andenvelopes having short rise and fall times. Although a burst seismicsignal 1 caused by a walking man: may include spectral components havingfrequencies. from approximately DC to something greater than 1 00 Hz,the amplitudes of the high frequency components having frequenciesgreater than 70 or 80 Hz are normally very low. The envelopes of seismicburst signals havev rise times that are typically shorter than 0.1second and durations having nominal values in the order of 0.215 second.Rain, helicopters and other aircraft produce: non-burst or continuousseismic vibrations of a much longer duration that result in continuousseismic signals that are characterized by spectral components extendingover a broad band of frequencies and envelopes having longrise and falltimes. The envelopes of" continuous seismic signals have rise times thatare generally longer than 1 second and durations that are typicallyseveral seconds long although such signals may extend for much longerperiods. Occasionally the envelopes of continuous signals may haveabrupt changes in amplitude that are generally spaced more than 1 0seconds apart. The amplitudes of the seismic signals on line 2 varyconsiderably depending on the proximity of a seismic source such as awalking man to the geophone.

Circuit 3 is an active filter comprising reactive elements and anamplifier having AGC circuit 4 connected between the output and an inputthereof. The amplifier increases the amplitude of low level signals online 2 so that they can be operated on by subsequent.

processing circuitry. The AGC changes the gain of the amplifier tomaintain a measure of the amplitude of the seismic signal essentiallyconstant regardless of the strength of a seismic signal I". This measuremay be the average or peak value of the. signal 1'. The output signal ofcircuit 3 is represented by the waveform 3' in FIG. 2B. This circuitalso causes the shapes of the envelopes of signals 1' and 3 to besimilar even though the amplitude of the former signal may varyconsiderably. The attack time of the AGC amplifier is,

. between footsteps which is approximately 0.8 second.

By way of example, the decay time may be 10 seconds. Filter 3 has a widefrequency passband for passing 7 energy difference strong low frequencyspectral components of a burst seismic signal I caused by a walking man.The lower cutoff frequency of filter 3 is selected to be high enough toblock extremely low frequency spectral components such as are. caused bythunder and to reduce the l/f noise of the system. By way of example,the lower cutoff frequency is preferably greater than 5 Hz. The uppercutoff frequency of this filter is selected to pass strong spectralcomponents in a burst seismic signal that is produced by a walking manbut to reduce the noise in the system. A burst seismic signal maycomprise strong spectral components having frequencies extending up to70 or 80 Hz. The upper cutoff frequency of filter 3 is thereforepreferably less than 100 Hz. In an embodiment of this invention whichwas successfully built and tested, filter 3 had a -40 Hz filter passbandand a rolloff of 6 db per octave.

Envelope detector 5 preferably comprises a fullwave rectifier forcausing all variations in the amplified signal 3' to be of the samepolarity (seewaveform 5" in broken lines in FIG. 2C) and a low passfilter on the output thereof for smoothing out the variations in therectified signal 5" and producing the envelope signal 5' (see the solidcurve in FIG. 2C). Alternatively, circuit 5 may be a half-wave rectifieror other nonlinear detector. The envelope signal 5' is comprised of aplurality of spectral components having different frequencies extendingdown to DC. I

It has been determined that the energy in spectral components of arectified burst signal 5 having frequencies between 1 and 4 Hz is muchgreater than that in the same spectral components of rectifiedcontinuous signals on line 6. The energies in spectral components ofrectified signals on line 6 having frequencies greater than 6 or 7 Hz,however, are approximately equal for both burst and continuous seismicsignals. These higher frequency spectral components therefore do littleto aid in distinguishing between burst and continuous seismic signalsand detecting a'man walking in a protected area. Rather, the presence ofthese spectral components tends to bias circuitry such that the statedin the aforementioned lower frequency spectral components on line 6 ismore difficult to detect. Also, there is very little energy from anysource of seismic signals in spectral components of a rectified signalon line 6 having frequencies of less I than 0.5 Hz. The-DC component ina rectified seismic signal on line 6 also biases circuitry to make thestated low frequency energy difference more difficult to detect. Filter7-is therefore preferably a bandpass filter having a lower cutofffrequency blocking the DC component of a rectified signal on line 6 andan upper custage filter wherein each stage comprises reactive andresistive elements and an operational amplifier. The structure, designconsiderations and operation of this filter are described in Swift SureDesign of Active Bandpass Filters," Electronic Design News, Jan. 15,1970, pp. 43-50.

Envelope detector 9 also preferably comprises a fullwave rectifier whichcauses all variations of the filtered signal 7 to be of the samepolarity (see the waveform 9" in broken lines in FIG. 2E) and a low passfilter on the output thereof for smoothing out the variations in therectified signal 9" for producing the envelope signal 9 (see the solidcurve in FIG. 25). Circuit 9anay also be a half-wave rectifier or othernonlinear detector. Integrator l 1 may be a simple resistor-capacitorintegrator preferably having a long integration time with respect to thetime constant of circuit 7. This integration time is preferably longcompared to the time between footsteps but less than the time requiredfor a man to walk through the detection range of geophone l. The timeconstant of integrator 11 may, by way of example, be 10 seconds. Thesignal stored by the integratoris represented by the waveform 11' inFIG. 2F. Alarm device 14 may, by way of example, be a bell or lamp.

The operation of the system will now be described in relation to thewaveforms in FIG. 2 which represent signals produced by associatedcomponents of the system in response to a walking man producing burstseismic vibrations in the earth. These vibrations are detected by thegeophone which produces the electrical burst seismic signals 1' (seeFIG. 2A) on line 2 that are amplified ,and filtered. Spectral componentsof the seismic signal 1 having frequencies within the l0-40 Hz passbandof filter 3 comprise the filtered signal 3 (see FIG. 2B). The shapes ofthe signals 1 and 3 are similar. The filtered signals 3' are rectifiedby detector 5 to produce the rectified and envelope signals 5" and 5,respectively, (see FIG. 2C) and are filtered by band pass circuit 7 toproduce the signal 7' (see FIG. 2D). Spectral components of envelopesignals 5 (see FIG. 2C) having frequencies within the I-4 Hz passband offilter 7 comprise the filtered signals 7' which are rectified bydetector 9. The energy in the rectified signals 9 (see FIG. 2E) aresummed by the integratorto produce the signal 11 (see FIG. 2F). If asufficient number of burst seismic signals 1 are present during the 10second integration time of circuit 11, the amplitude of the signal 11'exceeds the threshold level V, a

(see FIG. 2F) of circuit 12 and alarm device 14 is energized to indicatethat seismic signals 1' are burst seismic signals.

What is claimed is: ILApparatus for detecting intrusion of a protectedarea of a medium by discriminating between burst and continuous seismi'cvibrations produced in the medium, comprising a seismic sensorassociated with the medium and having an area of detection defining theprotected area of the medium, said sensor being responsive to seismicvibrations in the protected area of the medium for producing associatedseismic signals, means for passing a first signal having spectralcomponents 'of the seismic signals with frequencies greater than aprescribed low frequency limit and corresponding to frequencies ofstrong spectral components in a burst seismic signal produced by awalking man, said passing means comprising automatic gain controlcircuitry for maintaining a measure of the amplitude of the seismicsignal essentially constant,

means for converting the first signal to a second broadband signalcomprising spectral components having frequencies less than said lowfrequency limit,

means for detecting the energy in a band of low frequency spectralcomponents comprising the second signal, said frequency band of thedetecting means having a high frequency limit less than said lowfrequency limit of said passing means, and 7 means for indicating thatthe sensed seismic vibrations are burst seismic vibrations and intrusionof the protected area when said detected energy is greater than aprescribed threshold level.

2. Apparatus according to claim 1 wherein said der tecting meanscomprises a first filter having an upper cutoff frequency of less thanHz corresponding to the high frequency limit, a first 'envelopedetectoi'. responsive to the output signal of said first filter, and anintegrator summing the output signal of said first envelope detectorovera time interval that is long compared to the time between footstepsof a walking man.

3. Apparatus according to claim 2 wherein said passing means comprises asecond filter having a bandpass frequency response and a lower cutofffrequency of greater than 5 Hz corresponding to the low frequenfilter isa bandpass filter and has a lower cutoff frequency of approximately 1 Hzand an upper cutoff frequency of s 6 Hz.

7. A seismic signal processor for discriminating between burst andcontinuous seismic signals comprising v a first bandpass filterresponsive to a seismic signal for passing a firstsignal comprisingspectral components havingfrequencies corresponding to the frequenciesof strong spectral components of a burst seismic signal produced by awalking man, said first filter having a lower cutoff frequency ofgreater than 5 Hz, automatic gain control circuitry for maintainingessentially constant in the first signal a measure of the amplitude ofthe seismic signal, a detector having a nonlinear frequency responserelationship and being responsive to the first signal for producing asecond signal comprising low frequency spectral components havingfrequencies of less than the lower cutoff frequency of said firstfilter, means for detecting the energy in low frequency spectralcomponents of the second signal, and 1 a threshold circuit responsive tothe output of said detecting means for indicating that theseismic signalis a burst seismic signal when said energy is greater than a prescribedthreshold level. 8. The processor according to claim 7 wherein saiddetecting means detects the energy in spectral components of the secondsignal having frequencies less than the lower cutoff frequency of saidfirst filter.

9. The processor according to claim 7 wherein sa d detecting meanscomprises asecond bandpass filter responsive to the second signal andhaving a lower cutoff frequency of greater than 0.5 Hz and an uppercutoff frequency of less than 10 Hz, 7

a second detector having a nonlinear frequency response characteristicfor rectifying the filtered signal from said second bandpass filter, and

an integrator for summing the rectified signal over a time interval thatis long compared to the time between footsteps of a walking man.

10. The processor according to claim 9 wherein said second filter hasupper and lower cutoff frequencies of approximately 1 and 5 Hz,respectively.

1. Apparatus for detecting intrusion of a protected area of a medium bydiscriminating between burst and continuous seismic vibrations producedin the medium, comprising a seismic sensor associated with the mediumand having an area of detection defining the protected area of themedium, said sensor being responsive to seismic vibrations in theprotected area of the medium for producing associated seismic signals,means for passing a first signal having spectral components of theseismic signals with frequencies greater than a prescribed low frequencylimit and corresponding to frequencies of strong spectral components ina burst seismic signal produced by a walking man, said passing meanscomprising automatic gain control circuitry for maintaining a measure ofthe amplitude of the seismic signal essentially constant, means forconverting the first signal to a second broadband signal comprisingspectral components having frequencies less than said low frequencylimit, means for detecting the energy in a band of low frequencyspectral components comprising the second signal, said frequency band ofthe detecting means having a high frequency limit less than said lowfrequency limit of said passing means, and means for indicating that thesensed seismic vibrations are burst seismic vibrations and intrusion ofthe protected area when said detected energy is greater than aprescribed threshold level.
 2. Apparatus according to claim 1 whereinsaid detecting means comprises a first filter having an upper cutofffrequency of less than 10 Hz corresponding to the high frequency limit,a first envelope detector responsive to the output signal of said firstfilter, and an integrator summing the output signal of said firstenvelope detector over a time interval that is long compared to the timEbetween footsteps of a walking man.
 3. Apparatus according to claim 2wherein said passing means comprises a second filter having a bandpassfrequency response and a lower cutoff frequency of greater than 5 Hzcorresponding to the low frequency limit.
 4. Apparatus according toclaim 3 wherein said converting means comprises a second envelopedetector having a nonlinear frequency response characteristic. 5.Apparatus according to claim 4 wherein said first filter has a bandpassfrequency response and an associated lower cutoff frequency of greaterthan 0.5 Hz.
 6. Apparatus according to claim 4 wherein said first filteris a bandpass filter and has a lower cutoff frequency of approximately 1Hz and an upper cutoff frequency of < or = 6 Hz.
 7. A seismic signalprocessor for discriminating between burst and continuous seismicsignals comprising a first bandpass filter responsive to a seismicsignal for passing a first signal comprising spectral components havingfrequencies corresponding to the frequencies of strong spectralcomponents of a burst seismic signal produced by a walking man, saidfirst filter having a lower cutoff frequency of greater than 5 Hz,automatic gain control circuitry for maintaining essentially constant inthe first signal a measure of the amplitude of the seismic signal, adetector having a nonlinear frequency response relationship and beingresponsive to the first signal for producing a second signal comprisinglow frequency spectral components having frequencies of less than thelower cutoff frequency of said first filter, means for detecting theenergy in low frequency spectral components of the second signal, and athreshold circuit responsive to the output of said detecting means forindicating that the seismic signal is a burst seismic signal when saidenergy is greater than a prescribed threshold level.
 8. The processoraccording to claim 7 wherein said detecting means detects the energy inspectral components of the second signal having frequencies less thanthe lower cutoff frequency of said first filter.
 9. The processoraccording to claim 7 wherein said detecting means comprises a secondbandpass filter responsive to the second signal and having a lowercutoff frequency of greater than 0.5 Hz and an upper cutoff frequency ofless than 10 Hz, a second detector having a nonlinear frequency responsecharacteristic for rectifying the filtered signal from said secondbandpass filter, and an integrator for summing the rectified signal overa time interval that is long compared to the time between footsteps of awalking man.
 10. The processor according to claim 9 wherein said secondfilter has upper and lower cutoff frequencies of approximately 1 and 5Hz, respectively.